In recent years, various wireless systems, such as cell phones and wireless LANs, have become widespread, and therefore, there has been a fear of exhaustion of frequency resources. The trend is shifting toward more and more various wireless systems being mixed. In view of the exhaustion of frequency resources, a need is expected to arise for such an environment in which improvement in radio wave utilization efficiencies of wireless systems is essential.
Currently, attention has been given to a wireless communication technology called a cognitive radio technology. The cognitive radio technology, which is also referred to as a communication environment recognizing technology, is a technology for achieving the improvement in radio wave utilization efficiencies of the wireless systems by appropriately recognizing a surrounding radio wave utilization environment and flexibly selecting an optimal frequency band, radio channel bandwidth, modulation method, access method, and the like. The cognitive radio technology has been receiving attention as a technology that has a potential to become a core of new generation mobile communication systems.
The implementation form of cognitive radio is still unclear. However, among the researchers, there is a growing core concept that “a plurality of wireless communication systems can adaptively utilize resources of ‘time’, ‘frequencies’, and ‘space’”. In addition, in recent years, the technology for software-defined radio has been making progress, and hence there soon comes the emergence of devices capable of flexibly changing the frequency band, the modulation method, and the like.
The currently-conceived method of utilizing the cognitive radio technology is such a method that enables a wireless terminal to automatically recognize its surrounding radio wave utilization environment, such as a frequency congestion status and a radio wave transmission method, to determine a frequency, a transmission method, and the like so as to perform efficient and appropriate communication in that environment, and to switch links so as to use that frequency, transmission method, and the like.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-515120    Patent Document 2: Japanese Laid-open Patent Publication No. 2005-523616    Patent Document 3: Japanese Laid-open Patent Publication No. 2005-512423
The currently-conceived method of utilizing the cognitive radio technology is a method in which the terminal selects a frequency based on the surrounding radio wave condition that the terminal itself can detect as far as possible. However, with this method, it is impossible to recognize a frequency band that might become available in the future, and also, there exist frequency bands that cannot be recognized by the mobile terminal, such as the frequency bands used for an artificial satellite and public radio. In addition, there is a problem in that it takes a long period of time to establish connection due to the fact that the surrounding radio wave utilization environment is recognized and judged.